The invention relates to a stage for the amplification of high frequency signals (HF) or intermediate frequency (IF) signals.
The making of amplification systems working with HF or IF signals generally uses amplifiers whose input impedance is fairly low and well defined for it is necessary to match the input impedance of the amplifier with the output impedance of a generator connected to the input of this amplifier.
Furthermore, for the input stages of the amplifier (also called head stages) a low noise factor and a high compression point are sought to obtain the best amplifier performance characteristics. Indeed, the dynamic range of the signal applied to the input stage of the amplifier is limited, for the weak input signals, by the inherent noise of the stage and, for the strong input signals, by the non-linearities of the stage.
In bipolar technology, the structure most suited to this type of application is a common base input stage. In this structure, the input stage has a transistor mounted in a common-base connection. The input signal is applied to the emitter of the transistor, the base being biased by a fixed potential and the transmitter being biased by a constant current. The output signal is taken in the form of voltage at the collector of the transistor by means of a load resistor.
The common base input stage naturally has a low input impedance, a low noise factor and a wide passband. The weakness of this type of stage lies in its poor linearity. Despite this drawback, it is the common base assembly that has the best useful dynamic range.
It is known that the input impedance Zin, of the common base stage, in the form of small signals is: ##EQU1##
Where k is the Boltzmann constant (1.38 E-23 SI), q is the charge of the electron (1.6 E-19 Sl), T is the absolute temperature of the transistor and I.sub.CO is the current of the collector at rest.
The gain in voltage Av in terms of small signals is: ##EQU2##
V.sub.E1 being the input voltage of the stage,
V.sub.S1 being the output voltage of the stage,
and R.sub.c being the value of the load resistor connected between the collector of the transistor and a supply voltage VCC.
The expression of the output voltage as a function of the input voltage in terms of big signals is: ##EQU3##
V.sub.in being the input voltage in terms of big signals.
The output voltage varies exponentially with the input voltage. This clearly shows the poor linearity of this stage. The "linear" zone at input is equal to some kT/q, namely some tens of millivolts at most. Furthermore, the non-linear transfer characteristic generates harmonics and intermodulation at output.
Two known approaches are used to improve the linearity of the common base stage.
The first approach, known as &lt;&lt;degeneration&gt;&gt;consists of the addition of a resistor (degeneration resistor) series-connected between the input of the amplifier stage and the emitter of the transistor mounted in a common-base connection.
The input impedance is increased by the value of this resistor, leading to an increase in the idle current of the transistor, given that the input impedance of the transistor varies as the reverse of the idle current.
The linear zone at input of a stage of this kind is equal to about the range of the product of the current multiplied by the degeneration resistance plus some kT/q. It is therefore greater than that of the non-degenerate stage.
The tradeoff is that the noise factor is degraded. The increase in the permissible levels is lower than the degradation of the noise. The useful dynamic range is therefore degraded. This approach however is used when it is desired to increase the permissible levels and when the noise level is not of vital importance.
In the second approach, two transistors are mounted in differential form, each of the transistors being mounted in a common-base connection. In this configuration, the emitters of the transistors receive an input signal in differential form, the output of the stage being obtained in the same way in differential form at the collectors of the two transistors.
The making of the input stage in differential form enables the input impedance of the amplifier to be doubled. It is possible to double the idle current of each transistor if the same input impedance is to be preserved.
The linear zone at input is more than doubled as compared with the single stage comprising a single transistor, through the compensation for the curvature of the electrical parameters of one transistor by the other.
The noise factor is slightly degraded as compared with the stage being referred to. The increase in the permissible level is higher than the degradation of the noise. The useful dynamic range is therefore slightly improved. The transfer characteristic is an odd value, as a result of which the output spectrum contains few second harmonics and the intermodulation level is lower.
This stage in differential form, like the degenerate stage, is valuable for increasing the permissible level when the noise performance characteristics are not too important. The drawback of this stage is that it requires a signal in differential form, which is rarely the case in HF systems where the signal is often referenced with respect to the ground.
In the same way as in the case of a stage comprising a single transistor mounted in a common-base connection, the differential stage may be made degenerate by the serial connection of a resistor between each differential input of the stage and each emitter of the two transistors. This makes it possible to extend the range of the input linearity, but to the detriment of the noise factor. In this configuration, the permissible level of the input signal is increased but the useful dynamic range is thereby reduced.